This invention relates to guidance devices for self-powered vehicles and more particularly to a wire-following guidance device for an order picking vehicle.
In the material handling industry, high-rise order picker vehicles (OPVs) permit narrow aisle storage and retrieval operations of nonpalletized case or item storage. Such OPVs carry an operator on a lifting platform who picks orders from either a pallet or a storage module. The lifting platform incorporates the vehicle control so the operator can ride on the platform. The aisle widths are extremely narrow and may be as narrow as 4 feet. In the applicant's copending application the guidance system allows the operator to select between manual, power steering guidance of the vehicle or automatic guidance of the vehicle. In the automatic guidance mode, the vehicle follows an energized wire which is buried in the floor over which the vehicle travels.
In many self-guided vehicle systems, including the system described in the applicant's copending application referred to above, the vehicle has a pair of wheels on a fixed, that is, nonsteerable axle and a steerable wheel which is usually located in the front of the vehicle with respect to the normal direction of travel. The device for sensing the buried, energized wire then includes at least a pair of coils which straddle the wire and which are mounted on the vehicle ahead of the fixed axle. The purpose in mounting the coils ahead of the fixed axle is to obtain servo stability. If it is desired to move the vehicle in the reverse direction, in order to retain stability, it is then necessary to mount an auxiliary pair of sensor coils on the vehicle in a position such that they precede the fixed, nonsteerable axle when the vehicle travels in the reverse direction. See, for example, U.S. Pat. No. 3,667,564 (Schnell) which described an elaborate mechanism for having two pairs of steerable wheels, one of which is mechanically moved out of position when the vehicle is traveling in the forward direction and which is lowered to the ground and steerable when the vehicle travels in the reverse direction. The normal steerable wheel must be made to be nonrevolvable around its vertical axis. The reverse direction sensor coils are also lowered into and out of position depending on the direction of travel of the vehicle. This mechanism is rather complicated and clumsy. A further problem is that in the event a load is to be carried behind the fixed axle, that is, on the forklift itself, the load when it is lowered to the ground will interfere with the sensor coils positioned underneath the forklift and behind the axle.